HOME IMPROVEMENT : Secret to Stucco Patch Is to Approximate Original Mix

Whether it’s smooth and scored to look like stone, textured or pebble dashed, stucco is a decorative finish that adds interest to the interior walls of many old houses.

Many of the ingredients in early stucco aren’t available today or have changed over the years. Because of this, it’s nearly impossible to get an exact duplication of an original stucco. It is, however, possible to come close enough to restore minor patches of failed stucco. The trick is to try and match the original stucco mixture as close as possible in strength, composition, color and texture.

Homes built around the turn of the century usually have original stucco that is cement-based. A simple way to determine whether your stucco is cement or lime-based is to put a piece of the stucco in a glass of water. If it doesn’t stay firm and most of it dissolves and becomes mushy, it’s safe to assume you’re working with a soft, lime-sand stucco.

There are many recipes for this soft stucco. An early one calls for two parts sand and one part lime. Today, you’d add a little portland cement to this mix for workability, along with animal hair if the original had it. For those patching the harder cement stuccos, a mix with a high content of portland cement should be used.

Don’t be tempted to patch a soft, lime-based stucco with a harder portland cement-based stucco (or vice versa). They aren’t compatible. The materials’ differing rates of expansion and contraction will generally cause your construction work to crack.

In most cases, the spots where stucco has failed on your house walls will be obvious. But there are situations where the lath may have pulled away from the wall, causing bulges or bubbles in the outer surface. After a thorough visual inspection, walking around the house and lightly tapping with a rubber or wooden mallet, any spots where you suspect underlying failure may reveal hidden spots of damage that should be repaired.

Don’t try to tackle too large a spot at once. Make sure the underlying surface is clean and sound, and test your mixture in an inconspicuous spot. You might have to alter your mix a bit to get the best match. Make sure, too, that your patches match any existing control joints, which are breaks in the stucco that allow for shrinkage and expansion.

Stucco is commonly applied in three coats, although occasionally you’ll see walls that are only wearing two. The first coat, called the scratch coat, is generally one-fourth to three-fourths inch thick and is applied to metal lath, dampened wood lath or masonry. As its name implies, it’s scratched or roughened so the second coat adheres.

As soon as it’s dry--usually within 24 to 72 hours--the second or brown coat, which is usually the same thickness as the first, is applied. In a plain, two-coat application, this will be the final coat. More often, a third, or finish coat of stucco tops it.

If that’s the case, the brown coat, too, is lightly crosshatched to facilitate the final coat’s adhesion. It’s in this thin top coat--often only one-fourth-inch thick--that any texture or design is added. It’s applied after the brown coat sets.